EVOLUTION OF THE COLORS OP BIRDS. 205 



territory have dififered to such an extent that the sexual 

 organs are affected and they are no longer fertile inter 

 se* any mark by which they might distinguish their 

 own kind from the other stock, would serve in the per- 

 petuation of the race by preventing infertile crosses. 

 These different wing bars in the genus Dendragapus 

 may not improbably be characters of this sort. In the 

 band-tailed pigeon {Columba fasciata) the terminal third 

 of the tail is lighter and is separated from the basal por- 

 tion by a broad band of blackish. It may be in this 

 instance that the black bar was originally developed as 

 a discriminative mark, and that now, when no longer 

 needed for this purpose on account of the isolation 

 of the species from allied forms, natural selection 

 has turned it to account as a directive mark. In 

 the passenger pigeon {Ectopistes migrator ins) the tail 

 markings are highly complex. They are thus described 

 in Ridgway's Manual: " Shading from dusky on middle 

 feathers through gradually lighter shades of gray, to 

 white on outer webs of lateral pair, the inner web of 

 each feather (except middle pair) with a transverse 

 blackish spot, preceded by one of rufous." The neces- 

 sity for directive recognition marks would be especially 

 necessary in a species moving in such large flocks, from 

 which individuals or small parties would constantly get 

 astray. It may be that the complex pattern on the tail 

 feathers was evolved first as a discriminative mark, for 

 except in size, the species might easily have been con- 

 fused with the mourning dove (Zenaidura macroura) 

 which has the tail marked with a simple irregular sub- 

 terminal band of dusky. The white-fronted dove (En- 

 gyptila albifrons) has an especially characteristic form of 

 recognition marking on the tail, all but the middle pair 

 of feathers being black, broadly tipped with white. The 



*See discussion of physiological selection ante. p. 123. 



